Exercise training has been shown to decrease heart rate at submaximal workloads in humans and animals (dogs). The adaptation of the heart and coronary vascular system to exercise training may be the result of changes in autonomic nervous system balance, both at the sino-atrial node as this would affect heart rate and at the coronary vessels themselves. A change in the role of the sympathetic nervous system on myocardial cells must be also considered as this would relate to changes in ventricular volume and wall thickness. In chronically instrumented dogs trained by treadmill running, the role of the sympathetic nervous system on both coronary hemodynamics and ventricular volume will be examined. This will be accomplished before, during, and after the animal has been trained. The training program will encompass alternating daily periods of sprint and endurance running until the criterion of training has been achieved. The changes in ventricular volume and wall thickness will be related to the level of training in a temporal fashion and modified, if possible, by removal of portions of the autonomic nervous system. The coronary vascular reserve will be determined under similar conditions. BIBLIOGRAPHIC REFERENCES: Asimakis, G.K., Stone, H.L. and Sordahl, L.A.: Comparative studies of mitochondria and sarcoplasmic reticulum from exercise-trained and stellectomized dog heart. Physiologist 18(3):125, 1975. Young, S.D. and Stone, H.L.: Effect of a reduction in arterial oxygen content (carbon monoxide) on coronary flow. Aviation, Space & Environ. Med. 47(2):142-146, 1976.